The present invention relates to a transmission.
There has been proposed by General Motors Corporation in the United States a THM 700 type transmission which includes two planetary gear sets to provide four speeds. In this known transmission, there exists a member which will rotate at a speed higher than an allowable level during operation. Thus, poor durability of this member and its associated bearing structure is unavoidable.
The assignee of the present application has developed a transmission as shown in FIG. 5. This known transmission is described in"RN4F02A type, RL4F02A type automatic transaxle service manual" (A261C06). It does not have such a rotary member which rotates at a speed higher than an allowable level.
Referring to FIG. 5, the above mentioned known transmission comprises an input shaft 1, an output shaft 2, a front planetary gear set 3, and a rear planetary gear set 4. Rear planetary gear set 4 is arranged between output shaft 2 and front planetary gear set 3. A sun gear 3S of front planetary gear set 3 is adapted to be anchored by a band brake B/B and it is connectable via a reverse clutch R/C to input shaft 1. A carrier 3C rotatably supporting pinions 3P is connectable via a high clutch H/C to input shaft 1, and is operatively associated with a low one-way clutch LO/C which is arranged to prevent the carrier from rotating in the reverse or in the opposite direction to forward rotation of input shaft 1. It is adapted to be anchored by a low & reverse brake LR/B, and it is connectable via a low clutch L/C to a ring gear 4R of rear planetary gear set 4. Rear planetary gear set 4 has a carrier 4C connected to a ring gear 3R of front planetary gear set 3 and also to output shaft, and it has its sun gear 4S connected to input shaft 1.
With the known transmission, there are established four forward speeds and one reverse by activating a selected combination of friction elements in the pattern shown in the accompanying table illustrated in FIG. 6. In FIG. 6, friction elements which are activated are denoted by the reference character o, while friction elements which are not activated are denoted by x. III range designates a state in D range where establishment of an overdrive (OD) or 4th speed is prohibited.
In this transmission, low clutch L/C must be deactivated or released to interrupt the drive connection between carrier 3C and ring gear 4R in shifting to 4th speed (overdrive). Thus, in addition to activation or release of low clutch L/C, another friction element must be activated or released during a shift to 4th speed or during a shift from 4th speed. Thus, one of the two friction elements has to be activated in good timed relationship with release of the other in order to prevent the engine from racing and to prevent the transmission from interlocking.
Shift from 3rd speed to 4th speed is effected by release of low clutch L/C and engagement of band brake B/B. If band brake B/B is engaged in good timed relationship with release of low clutch L/C as indicated by phantom line and fully drawn line in FIG. 3(a), no substantial shocks take place during the shift as will be appreciated from variation in engine speed and variation in transmission output shaft torque which are illustrated by fully drawn lines. If the engagement timing of band brake B/B is delayed as illustrated by broken line in FIG. 3(a), the engine races as illustrated by broken line designated by the reference numeral a. The inertia of rotation caused by this engine racing results in a peak torque, in transmission output shaft torque, illustrated by broken line designated by the reference numeral b upon engagement of band brake B/B.
Shift from 4th speed to 3rd speed is effected by engagement of low clutch L/C and release of band brake B/B. If low clutch L/C is engaged in good timed relationship with release of band brake B/B as illustrated in phantom line and fully drawn line in FIG. 3(b), no substantial shocks take place during the shift as will be appreciated from variation in engine speed and variation in transmission output shaft torque which are illustrated by fully drawn lines. If the release timing of band brake B/B is delayed as illustrated by broken line in FIG. 3(b), there occurs temporal interlock in the transmission which causes engine speed to vary as illustrated by the broken line. This variation in engine speed causes a drop c in transmission output shaft torque and subsequent peak d in transmission output torque due to reaction of the drop c. This peak d causes substantial shocks during the shift.
Shift from 4th speed to 2nd speed is effected by engagement of low clutch L/C and release of high clutch B/C. If low clutch L/C is engaged in good timed relationship with release of high clutch H/C as illustrated by phantom line and fully drawn line in FIG. 3(c), no substantial shocks take place during the shift as will be appreciated from variation in engine speed and variation in transmission output shaft torque which are illustrated by fully drawn lines in FIG. 3(c). If the release timing of high clutch H/C is delayed as illustrated by broken line in FIG. 3(c), there occurs temporal interlock in the transmission which causes engine speed to vary as illustrated by broken line. This variation in engine speed causes a drop e in transmission output shaft torque and subsequent peak f in transmission output shaft torque due to reaction caused by the drop e. This peak f causes substantial shocks during the shift.
For alleviating shocks during shifting operation, it has therefore been necessary to effect precision control of timing of change and rate of change of hydraulic fluid pressure which is used to effect engagement/release of low clutch L/C and release/engagement of friction element which is to shift in timed relationship with the action of low clutch L/C.
An object of the present invention is to provide a transmission which is free from the above mentioned problems.